The present invention relates to an electrophotographic recording material composed of an electrically conductive layer support, which is suitable, in particular, for the production of printing forms or printed circuits, and of a photoconductive layer comprised of an organic photoconductor, a binder and a sensitizer.
Recording materials for reproduction by electrophotographic means are known. They are, for example, employed in electrophotographic processes in which a photoconductive layer which consists of or contains organic or inorganic photoconductors, is electrostatically charged in the absence of actinic radiation and subsequently imagewise exposed. Thereby, the electric charge flows off in the areas which are struck by light, and the image thus produced is made visible by means of a developer, is transferred, if appropriate, and is fixed.
Processes for the production of electrophotographic recording materials and for the filmless production of printing forms are widely used today. Apart from a good sensitivity to light, the photoconductive layer must exhibit low discharge in the dark and a low sensitivity to pre-exposure, and the known layers do not always possess these properties to a sufficient degree.
A great number of organic compounds which possess more or less satisfying photoconductor properties and which can be used in the production of layers for electrophotographic purposes are known. Compounds which possess less satisfying photoconductor properties include, for example, the condensation products of an aromatic o-dicarboxylic acid anhydride and an aromatic amine which are disclosed by German Pat. No. 12 17 785 (equivalent to U.S. Pat. No. 3,290,146). Compounds which possess good photoconductor properties include, for example, the oxadiazole derivatives described in German Pat. No. 10 58 836 (equivalent to U.S. Pat. No. 3,189,447).
It is also known to use mixtures of several photoconductors in electrophotographic recording materials. For example, German Pat. No. 12 17 785 describes the addition of a substituted oxadiazole to a photoconductive layer comprising a condensation product of phthalic acid anhyride and diethyl aniline. This improves the light sensitivity of the layer.
In the production of printing forms or printed circuits by electrophotographic means it is of crucial importance that in addition to a good light sensitivity the photoconductive layer also possesses good resolution properties and allows for a correct gradual reproduction of screen density values of screened images. In the production of printing forms, process cameras are employed which, due to a special lens, make it possible to expose even large formats of, for example, 680 mm.times.500 mm, in such a way that even in the marginal zones fine structures, such as thin lines or screen dots, are exactly reproduced. To ensure a sharp image reproduction, the light beams falling in through the lens have to be limited by a shutter, which makes longer exposure times necessary. The exposure times employed in current practice vary between about 30 and 50 seconds. This means that during this time the charge must not flow off in the non-exposed areas, i.e., a photoconductor layer suitable for the production of high-quality printing forms must not be conductive in the dark.
Furthermore, when printing forms are produced by electrophotographic means, the photoconductor layer should be soluble in aqueous-alkaline solvents, which requires the use of appropriate binders.
When the highly light-sensitive oxadiazoles are used in combination with alkali-soluble binders, it is found, however, that discharge in the dark occurs to such an extent that, after exposure by means of a process camera, it is no possible to develop fine lines and screen dots.